The mammalian superior olivary complex (SOC) consists of three well defined cell groups, the medical superior olive, lateral superior olive and medical nucleus of the trapezoid body, which have identified roles in sound localization. These principal nuclei are surrounded by a sphere of accessory cell groups, collectively known as periolivary nuclei (PON). To a large extent, the PON are involved in the descending auditory pathway and participate in reflexive and feedback functions of the SOC. This application focuses on a specific periolivary region known as the superior paraolivary nucleus (SPON) in rodents (the dorsomedial periolivary nucleus DMPO, in non-rodents). This cell group is the largest of the PON in many species, approaching the size of the principal nuclei, but has received relatively little attention. By all accounts, the SPON appears to be a very complex structure, both in terms of its morphological and neurochemical characteristics and in terms of its participation with various aspects of the auditory pathway. By virtue of its relationship with other nuclei in the SOC, the SPON may play a role in binaural processing. The SPON is also a site of convergence of ascending and descending auditory projections, and may serve an important role in modulating the activity of descending systems. The long term aim of this research is to provide a better understanding of the functional role of this cell group. The proposed study combines classical neuroanatomical preparations with immunocytochemistry, electron microscopy an pathway tracing to perform the most comprehensive study to date of the SPON of the rat. A systematic strategy, involving increasingly detailed levels of analysis, will guide the project to its goal of providing a precise, quantitative characterization of the nucleus and its connectivity. The experiments will also uncover the topographic organization of the nucleus. This research will contribute to a better overall understanding of the auditory brainstem. in the long term, such information will be critical to the design of prosthetic devices that may replace non-functioning parts of the central auditory system.